Simple refrigeration systems use a method called vapor compression cycle. The vapor compression cycle is a method in which a compressor such as a piston compressor compresses a low-pressure refrigerant vapor. In a next stage, a condenser condenses the warm compressed vapor, resulting in a partial or complete condensation of the vapor. This condensed refrigerant then passes through a fine capillary tube or other constriction into a larger chamber at low pressure. As the refrigerant enters the larger chamber, it evaporates and absorbs heat, resulting in the vapor. This refrigerant vapor is then routed to the intake of the compressor, thus closing the cycle. This is the so-called closed loop refrigeration system.
A single stage refrigerant system is used to achieve temperatures up to −40 degree C. For temperatures in the range −50.degree C. to −80.degree C. a two-stage cascaded vapor compression system has been proposed. This method uses a single refrigerant per stage and two compressors, one for each stage. For still lower refrigeration temperatures, more cascaded stages are required. Typically, four stage cascade vapor compression systems are proposed for reaching cryogenic temperatures, lower than −150 degree C.
An advance in technology has been achieved by using a single compression system with a composition of refrigerants. This method has been used to achieve temperatures far below those that can be achieved using a cascaded multi-stage system, for example, the range −100 degree C. to −210 degree C. This method uses a composition of several refrigerants each of which with different boiling points.
The low temperatures can be achieved using different process systems, some employing one or more phase separators and some that don't employ any phase separators. Systems that employ phase separators are commonly known as multistage mixed refrigerant systems, and those that don't employ phase separators as single stage mixed refrigerant systems. In all the systems, the high pressure refrigerant is cooled to a low temperature and expanded to a low pressure to provide the necessary refrigeration to cool a load. The low pressure refrigerant leaving the evaporator provides the necessary refrigerant to cool the high pressure refrigerant to low temperatures.
Phase separators are employed to remove some of the high boiling point components and lubricating oil carried over from the compressor from reaching low temperatures where they may freeze. In the phase separators, the liquid consists mostly of the high boiling point components, and the vapour phase preferably does not contain any high boiling components. The liquid is expanded to a lower pressure, mixed with the low pressure refrigerant returning from the evaporator at an intermediate temperature and returned to the compressor. The expanded liquid also provides the necessary cooling to coo: and condense the vapour stream leaving the phase separator. The use of phase separators also allow balancing the refrigeration needed to cool the high pressure refrigerant at different temperatures.
Sometimes part of the refrigeration needed to cool the high pressure refrigerant is provided by another refrigerator or a cold stream. Such systems are known as precooled systems. Precooling can be used in both single and multistage mixed refrigerant systems. Precooling essentially allows the system to operate at nearly same efficiency at all ambient temperatures. The use of phase separators reduces the oil carried over to the low temperatures and improves the reliability of the system. The use of both phase separators and precooling improves the system reliability considerably by removing part of the oil carried over at low temperatures. The present invention relates to a refrigerant composition for multistage refrigeration systems precooled to a temperature at least 20 K below the ambient temperature, and more preferably precooled to a temperature of 230 to 260 K.
U.S. Pat. No. 5,441,658 describes a mixed refrigerant cycle with out phase separators. The patent claims the following mixture:    30 to 50 molar percent of nitrogen    at least some but less than 20 molar percent of methane    more than 30 molar percent of propane    and balance ethylene or ethane.
U.S. Pat. No. 6,513,338 describes a precooled mixed refrigerant cycle, but with out any phase separators. The patent claims the following mixture:    0.06 mol/mol to 0.20 mol/mol propane,    from 0.26 mol/mol to 0.36 mol/mol nitrogen and    from 0.20 mol/mol to 0.38 mol/mol methane,    the remainder being ethane.
As would be known to a person skilled in the art, the refrigerant composition depends on the type of system employed, the temperature at which precooling is done and whether phase separators are used or not. In a system with out phase separators, the refrigerant composition leaving the compressor is cooled in its entirety to the refrigeration temperature. On the other hand, in phase separator systems, part of the refrigerant returns back to the compressor at temperatures much above the refrigeration temperature. The refrigerant composition compressed by the compressor in phase separator systems therefore needs to be different from that used with out phase separators.
This disclosure is directed toward a composition of refrigerants for use in precooled multi stage mixed refrigerant systems to provide refrigeration below 110 Kelvin. Applications for such composition of refrigerants include household or commercial refrigeration systems such as refrigerators, freezers, electronic circuit cooling, medical applications, cryo-vacuum pumps, storing of biological specimens and tissues at low temperatures, cooling of Gamma-ray, Infra Red and X-ray detectors, cryosurgery and the like.